The electrodischarge (EDM) and electrochemical (ECM) machining processes have been developed in recent years primarily to overcome the problems associated with hard, heat-resistant alloys which are difficult to machine by traditional methods. The former process removes metal by the erosive effect of electric sparks, discharged across a dielectric, whilst the latter relies on electrolytic dissolution from an anodic workpiece, the gap between that electrode and the cathode tool being filled with electrolyte. Although EDM yields higher dimensional accuracy than ECM, it is a slow process and the workpiece is normally left with its surface layers metallurgically damaged by the formation of a heat-affected zone. On the other hand, ECM gives a lower dimensional accuracy. This process is usually capable of higher metal removal rates than EDM and it leaves hardly any deleterious effect on the surface properties of the material; indeed, a smooth bright surface finish is a common feature of ECM.
In shaping a workpiece by ECM the workpiece and tool are made the anode and cathode, respectively, of an electrolytic cell and a potential difference is applied across the electrodes. The rate of dissolution of metal from the anode is approximately in inverse proportion to the distance between the electrodes, as a consequence of Ohm's law.
As ECM proceeds, and with the cathode-tool being driven towards the anode, usually at a constant rate, the gap width along the electrode length gradually tends to an equilibrium value, typically 0.5 mm. Under those conditions, a shape roughly complementary to that of the cathode is reproduced on the anode workpiece. Since the only electrolytic reaction at the cathode is gas evolution, there is no tool wear.
Some of the main features of EDM are now mentioned briefly. The electrode and workpiece are separated by a small gap, typically 100 .mu.m, filled with dielectric fluid such as paraffin or light oil. An applied voltage, usually about 80 V, is applied across the gap. Current flow of the order of 1 mA results on the formation of a dielectric vapour bubble, due to Joule heating. The presence of the gas vapour plays a significant part in the sparking action in EDM. After an "ignition delay", typically of about 0.1 to 5 .mu.s, breakdown occurs. Sparking then takes place across the inter-electrode gap but in order to prevent arcing, the voltage is removed after a short interval. A further short interval is then allowed to elapse before the next voltage pulse so that the fluid in the inter-electrode gap can de-ionise.
The consequence of a series of voltage pulses applied across the gap is the production of a set of random discrete discharges. The discharges affect both anode and cathode electrodes, causing the local temperature to rise to about 4,000 to 10,000 K. This intense heat at the electrodes results in metal removal by vaporisation.
Although the discharge clearly affects both electrodes, judicious choice of process conditions and tool materials can reduce wear of the electrode to as little as one percent of the workpiece. Common electrode materials which satisfy the criterion for low tool wear are high density graphite, copper and copper tungsten.